Reliability is one of the major requirements for power and opto-electronic devices across all segments. High operation temperature and/or high thermomechanical stress cause defects and degradation of materials and interconnects, which may lead to malfunctions with costly or even life-threatening consequences. To avoid or at least reduce failures, nondestructive testing (NDT) methods are common within development and production of power and opto-electronics. Currently, the dominating NDT methods are X-ray, scanning acoustic microscopy (SAM), and transient thermal analysis (TTA). However, they have different strengths and weaknesses with respect to materials and mechanical designs. This paper compares these NDT methods for different interconnect technologies, i.e., reflow soldering, adhesive, and sintered interconnection. While X-ray provided adequate results for soldered interfaces, inspection of adhesives and sintered interconnects was not possible. With SAM, evaluation of adhesives and sintered interconnects was also feasible, but quality depended strongly on the sample under test. TTA enabled sufficiently detailed results for all the interconnect applications. Automated TTA equipment, as the in-house developed tester used within this investigation, enabled measurement times compatible with SAM and X-ray. In the investigations, all methods revealed their pros and cons, and their selection has to depend on the sample under tests and the required analysis depth and data details. In the paper, guidelines are formulated for an appropriate decision on the NDT method depending on sample and requirements.

A new method is developed to crystallize tin formates in a formic acid-enriched nitrogen atmosphere. The SAC alloy is used.

To solder metal parts efficiently, layers of oxides on the surfaces need to be removed. The use of formic acid vapour as a reducing agent is a valid alternative to liquid fluxes, i.e. organic or inorganic based. Fluxless soldering is growing in its applicability, especially in microelectronics packaging. Nevertheless, some residues are produced. This study focuses on the characterization of these residues, which are obtained by flowing nitrogen gas enriched with formic acid at 200 °C on Sn₃Ag_0.5Cu (SAC305) metal alloy, a widely used soldering material. The formation of microcrystals is observed and attributed to heat and mass transfer processes of the formic acid vapor. While there is a wealth of reported studies on soldering results under a formic acid atmosphere, there is a lack of information with regards to a specific understanding of the chemical compositions of these undesired residues. The second purpose of this study was to confirm the recently introduced method of producing Sn-based crystals under a formic acid atmosphere. X-ray diffraction (XRD), energy-dispersive X-ray (EDX) and infrared spectroscopy (IR spectroscopy) investigations allow us to assign the composition of the microstructures. Crystals of Sn formate, Sn oxide, and bimetallic Cu₆Sn₅ were detected and a mechanism of formation of these crystals was suggested.

This paper introduces a new measurement method for noise suppression in transient thermal analysis (TTA) which is used to measure the transient thermal impedance of power semiconductors and LEDs. An additional short pulse sequence is added to the standard TTA measurement procedure. The thermal response of this additional sequence is converted to the original single pulse response of standard TTA by superposition of previous pulses in the sequence. Applying this method, the noise before 1 ms can be significantly reduced without averaging over multiple measurement repetitions that would increase the measurement time accordingly. The noise reduction for times below 1 ms is important to resolve the thermal resistance layers close to the junction of a semiconductor. The measurement method and data analysis algorithm are developed for TTA measurements, but the principle is suitable for all linear time invariant (LTI) systems where the system response is measured over several time decades. The algorithm was evaluated on simulated and experimental data of a power LED. With an additional sequence of only 100 ms, a signal to noise ratio (SNR) improvement of 15.1 dB for simulative and 14.7 dB for experimental data was achieved. This is equivalent to averaging over 30 repetitions in the standard TTA procedure. Therefore, the new method achieves a reduction of measurement time of 97 % for applications where low noise levels are required for times below 1ms.